The present invention relates to an electronic control system for a carburetor of an internal combustion engine and more particularly to a means for controlling the air-fuel ratio of a mixture to a proper value during the warm-up operation of the engine.
Closed loop control systems for controlling the air-fuel ratio are known in the internal combustion engine emission control system art with the three-way catalyst. In one of such systems, the oxygen sensor for sensing the oxygen content of the exhaust gases and an electronic control means are provided for controlling an on-off electro-magnetic valve according to the signal from the oxygen sensor to provide the stoichiometric air-fuel ratio. The output voltage of the oxygen sensor varies according to the temperature of the sensor device. More particularly, when the temperature is below 300.degree. C., the output voltage is too low to operate the electronic control means for controlling the air-fuel ratio.
In a conventional system, the duty ratio (the ratio of the duration of valve open period to one on-off cycle of the on-off electro-magnetic valve) is fixed to a predetermined ratio during cold engine operation for providing a lean air-fuel mixture, and, on the other hand, an automatic choke device is provided to correct the lean air-fuel mixture to a proper air-fuel ratio according to the engine temperature for improving the operability of the cold engine.
The automatic choke device is adapted to close the choke valve by a bimetal element according to the engine temperature so as to start the engine in the cold and to progressively open accordingly as the temperature rises. In the auto-choke device, a slight variation in flow area of the choke valve causes a great variation of the air-fuel ratio. Therefore, it is difficult to control the air-fuel ratio to a desirable value by the automatic choke device.